The invention relates to an X-ray diagnostic installation comprising an image intensifier television chain which exhibits an x-ray image intensifier, an optical coupler, a television pickup tube, a central unit for controlling the scanning beam of the television pickup tube, a video amplifier for the picture signal, an outlet-connected image memory, which is likewise controlled by the central unit, and a monitor. Diagnostic installations of this type are employed for the purpose of transilluminating patients predominantly in the case of intermittent fluoroscopy, or for the purpose of preparing electric immediate (or no-delay) images; i.e., individual images.
In the case of a known X-ray diagnostic installation of the type initially cited (book "X-Ray Television" by A. Gebauer, 2nd Edition, Georg-Thieme-Publisher, Stuttgart, 1974, pages 53 and following), there is connected, between the X-ray image intensifier and the television pickup tube, for example of the vidicon type, an optical lens system which makes it possible, by means of stopping down, to adapt in its brightness the image to be picked up by the television pickup tube such that the television pickup tube is not overdriven, i.e., such that the charge potential of the target of the television pickup tube can be completely reversed in charge by the scanning beam.
In the case of the coupling of the television pickup tube to the X-ray image intensifier by means of a fiber optic system (book by Gebauer cited above, pages 68 and following), which is already in frequent use today, the possibility does not exist of stopping down by means of a mechanical diaphragm. Here, other approaches must be embarked upon for the purpose of adaptation of the brightness of the X-ray image to the television pickup tube.
A known possibility (see the book "Measurement Technique" of W. Dillenburger, 1972, published Schiele & Schon, Berlin, page 236) is an increase in the beam current intensity of the scanning beam, which, however, can proceed only up to a limit, the maximum current intensity, since otherwise the distortions increase too greatly. If the maximum beam current intensity is not sufficient for the complete charge reversal of the target, then the television pickup tube becomes overdriven and delivers an X-ray image which is too bright and poor in detail. In order to prevent overdriving, the X-ray dose rate must be reduced. Since, however, the quantum noise in the X-ray image has a more conspicuous and interfering effect the lower the dose rate, the quality of the X-ray image is thereby considerably reduced.
Recently, light flux diaphragms have been employed for fiber-optics (German Patent Application No. 2,846,295), which are comprised of a layer which is electrically controllable in its transparency. An adaptation of the brightness of the X-ray image to the following television pickup tube is hereby, indeed, possible. However, the arrangement of such a diaphragm in the optical path of rays and the electric control necessary to this end are very costly.
In the case of individual image operation of a television pickup tube, gamma-distortions in the scanning occur. In the case of dark image portions the brightness differences appear compressed. This effect is determined by the brightness-dependent charge reversal inertia (or lag) of the television pickup tube which is clearly greater for dark image portions, which generate only a low charge potential on the target, than for bright image portions. This effect occurs most strongly in the first scanning. Also, the electric signal-to-noise ratio for small signal levels is thereby smaller, so that an electronic correction of the gamma-distortion does not appear to make good sense.